This invention concerns a rotodynamic pump for varying output flow, for example suitable for recirculation of drilling fluid and transport of drill cutting from an underwater drilling operation to a separator on a surface drilling rig or similar. Among other things, the following is characteristic of such pumping operations:                The output flow varies quickly and frequently, also including regular full stops for making-up a drill string.        Frequently a disproportionately large lifting height relative to the output flow, as viewed from an ideal position for rotodynamic pumps.        Transport of drill cuttings with a varying size and hardness, and at the risk of random inclusions of rocks sized up to ˜Ø 50-75 mm.        Return of the drilling fluid is equally important to the return of cuttings, the mixing ratio of solids-liquid is typically 1-5% as defined by the application, and cannot be adapted to the pump.        The lifting height must be maintained approximately at full height during full stop in the output flow. Possible backflow of cuttings over time when at full stop in the output flow must not cause clogging or in any other way complicate a fast re-establishment of a full output flow.        The temperature must not quickly become critically high at a low output flow and a large lifting height.        The drill cutting must not become finely distributed by the pump so as to render it difficult to separate.        The pump must be able to operate continuously without interruptions for maintenance over periods of a few days to several months.        The drilling fluid will exhibit large variations in density and viscosity.        The weight of the system, which includes the motor, regulator, power supply, hoses, pipes and cables, is critical.        Non-scheduled maintenance is to be carried out effectively offshore.        
Thus far, disc pumps have essentially been used for the purpose, for example as described in U.S. Pat. No. 4,940,385. In principle, this concerns centrifugal pumps wherein the impeller consists of discs without blades, but with certain ribs or recesses contributing to accelerate the liquid in the best possible manner by means of shear forces. Among other things, the absence of blades offers the advantage of solid particles obtaining a considerably lower tangential velocity than the liquid, whereby erosion is reduced in both the disc and the pump housing. However, the efficiency and lifting height is reduced considerably relative to typical centrifugal pumps with blades. This is of particular relevance when pumping a liquid having low viscosity. Pumps of this type are suitable for high-viscosity liquids.
It may appear obvious to look onto the mining industry to find a pump design suitable for the above-mentioned application. Here, however, the lifting height requirement is normally lower, and the output flow requirement is even higher. The medium of pumping is usually water at volumes freely adaptable to the requirement of the pump. Common solutions involve large and heavy centrifugal pumps with moderate rotational speeds, but nevertheless with a higher specific velocity than what is possible to accommodate specific lifting height requirements. Typically, these pumps have heavy-duty, hard-wearing blades with a low pitch angle. With respect to the largest solid particle which is allowed to pass, the size of the pumps oftentimes makes the consideration less restricting concerning the optimization of the number and width of blades. The concentration of solid particles is high in these pumps, and a slurry having 20-30% of solid particles is typical. The high concentration of solid particles causes a lesser extent of heavier particles being hurled out at a high radial velocity toward the walls of the pump housing, which is due to the individual particle's freedom of movement, relative to the main flow, becoming more restricted. These “material pumps” are indeed heavily exposed to erosion and abrasion, but they may possibly be less exposed to situations where singular, heavy, hard and sharp particles hit the walls of the pump housing hard enough to cause e.g. a surface coating, such as tungsten carbide or similar, to become crushed or disintegrate into flakes.
It is commonly recognized that in order to achieve a high efficiency in a centrifugal pump, among other things, it is of advantage to shape the pump housing as a volute casing having, across the circumference, a gradually increasing cross-sectional area of flow toward the outlet, whereby the flow of liquid discharging from the periphery of the impeller may be distributed evenly across the circumference, and at a tangential velocity adapted to the rotational speed of the impeller and the profile of the blades. Usually—but not always—the entire length of the central axis in the cross-sectional area of flow of the volute casing lies in the same plane as a circle envisaged along the periphery of the impeller, and in the middle of the cross-sectional area of flow thereof.
When a volute casing is to be designed, however, the starting point must be a given output flow, a given impeller design, and a given rotational speed. A particular lifting height for the pump is also associated with these conditions. These design criteria correspond to what is termed as the pump's BEP—“best efficiency point”.
For a pump having constantly varying operating conditions—for example at regular periods at a sustained lifting height and no output flow—any choice of a volute casing design will be less than optimum during larger or smaller parts of the operating time. The flow of liquid leaving the impeller and flowing through the volute casing toward the outlet will, in cases of very low flow output, suddenly experience a virtual “wall” having a relatively large cross-section at the outlet. This results in strong turbulences, efficiency losses, erosion on the tongue at the outlet from the pump housing, local backflow into the impeller with subsequent erosion on the blades, and high pressure differences and vibrations across the circumference, which in turn inflicts large loads on the radial bearings of the impeller. There will also be a danger of critical heating in the pump.
In a disc pump in accordance with the above-mentioned U.S. Pat. No. 4,940,385, the disadvantages of operation outside BEP are reduced by virtue of the pump housing being of a cylindrical shape and having the same axis as the impeller, however arranged in a manner allowing the liquid to discharge from the pump housing through a rectilinear outlet at the periphery of the pump, and in a plane perpendicular to the axis of rotation and centred in the pump housing. Under no operating conditions will a pump having such a design achieve as high efficiency as what a corresponding centrifugal pump having a volute casing will achieve at around BEP, but the efficiency as well as the radial forces stabilize, in many cases, at an acceptable level within a window of operation. A cylindrical pump housing like this, however, will inflict new disadvantages if combined with a typical impeller having blades that divide the internal volume of liquid in the impeller into clearly separated masses, and where substantial throughput only is possible between the two blades passing at any time closest to the tongue at the pump outlet. By virtue of such a design, the throughput in the impeller will have to occur in bursts and constantly move between different blades.
The object is achieved by virtue of features disclosed herein.
Accordingly, the present invention may set forth to combine the best virtues of a disc pump having a cylindrical pump housing on one side, and a centrifugal pump having impeller blades and a volute casing on the other side, and combine considerations with respect to the partly contradictory requirements mentioned above in a better way than what has, thus far, been possible with known technology.
The object is achieved by virtue of features disclosed in the following description and in the subsequent claims.
A rotodynamic pump for varying output flow is provided, which is characterized in that in all cross-sections, which are vertical to the axis of rotation between axial outer positions for cross-sectional areas of flow at the periphery of the impeller, the inner wall of the pump housing forms approximately circular profiles being mainly concentric and having a continuously increasing radius from one toward the other one of said axial outer positions, and wherein a tongue, which truncates the outlet or diffuser of the pump from the annulus of the pump housing, does not contact said circular profiles between said axial outer positions.
The rotodynamic pump may comprise that the medium is conducted out of the cavity of the pump housing through a pump outlet with a cavity that cuts through the inner wall of the pump housing at the periphery on the side of the axial extent of the impeller where the radius of the inner wall of the pump housing is the largest.
The rotodynamic pump may comprise that the pump outlet cuts through the inner wall of the pump housing in an annulus, which is partly shielded from those parts of the cavity of the pump housing located closest to the impeller, and through a circular wall which, between the annulus and the impeller, extends radially outwards along the periphery of the impeller and along the inner radius of the annulus, however without cutting off the liquid communication between the impeller and the annulus.
The rotodynamic pump may comprise that the pump housing has a demountable front plate with a radius being marginally larger than the impeller, wherein the front plate is arranged in both axial and radial directions within the annulus, wherein seals are arranged between the front plate and other parts of the pump housing, and wherein the front plate is locked in an axial position by means of radial displacement of locking dogs extending outwards and into adapted recesses in the inner external wall of the annulus.
The rotodynamic pump may comprise that interchangeable front plates are individually integrated with various pipe bends forming the suction nozzle of the pump, and wherein the front plate with a pipe bend is capable, during mounting, of being rotated about the axis of rotation of the pump, and in any direction relative to the outlet, at least before it is locked down with the locking devices.
The rotodynamic pump may comprise that at least one selectable front plate has a pipe bend terminated with a flange adapted to corresponding flanges on the outlets of corresponding pumps, whereby two or more corresponding pumps are capable of being connected directly together, in series, in one or more compact ways without use of further transition pipes, bends or hoses.
The rotodynamic pump may comprise that the medium is conducted out of the pump housing through a channel shaped as a volute casing and positioned, in its entirety, outside the axial border positions for the cross-sectional area of flow at the periphery of the impeller, and wherein the centre line in said channel forms a helical line having an increasing distance from the axis of rotation, as viewed in a co-current direction, and an increasing axial distance from a motor toward the suction side of the pump.
The rotodynamic pump may comprise that it is equipped with an impeller of the disc-type, wherein two or more discs are held together only by small spacers, and wherein the internal side surfaces of the discs may be equipped with grooves in order to increase entrainment of liquid, however such that solid particles mainly are accelerated tangentially only due to flow resistance in the liquid and hence obtain a lower tangential velocity than the liquid.
The rotodynamic pump may comprise that it is equipped with an impeller having blades arranged in pairs, wherein the first blade in each pair starts at the smallest radius practically possible for free passage of the largest solid particle to be pumped, wherein said first blade has a low pitch angle, and wherein the second blade in each pair starts at a considerably larger radius than the first blade and is positioned, as viewed in the direction of rotation, in front of the first blade at such a distance that the largest and heavier particle to be pumped is allowed to pass underneath, and wherein the pitch angle of the second blade in each pair is considerably larger than that of the first blade in each pair.